Annual report pursuant to Section 13 and 15(d)

Asset Retirement Obligations - Reconciliation of Asset Retirement Obligations Liability (Details)

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Asset Retirement Obligations - Reconciliation of Asset Retirement Obligations Liability (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Asset Retirement Obligation, Roll Forward Analysis [Roll Forward]      
Asset retirement obligations, beginning of period $ 334,438 $ 378,322  
Asset retirement obligation settlements (72,409) (72,320) $ (32,555)
Accretion of discount 17,172 17,571 20,703
Liabilities incurred 163 398  
Revisions of estimated liabilities 21,082 10,467  
Asset retirement obligations, end of period 300,446 334,438 $ 378,322
Less current portion 23,613 78,264  
Long-term $ 276,833 $ 256,174